The invention relates to a low application temperature hot melt adhesive compositions that possess high heat resistance. The adhesive is particularly well suited for automated assembly applications, e.g., case, carton and tray manufacture.
Hot melt adhesives are widely used for various commercial applications such as product assembly and packaging, including case sealing and carton closing operations. Such hot melt adhesives are applied to a substrate while in its molten state and cooled to harden the adhesive layer.
Most commercially available hot melt adhesives require temperatures of 350xc2x0 F. (1770xc2x0 C.) or greater to ensure complete melting of all the components and also to achieve a satisfactory application viscosity. The need for such elevated temperatures is not without problems. The high temperatures increase operator risks with respect both to burns and to inhalation of residual volatiles. In addition, use of high temperatures require more energy, placing greater demands on the manufacturing facility. While adhesive formulations that can be applied at temperatures below 300xc2x0 F. (151xc2x0 C.) can be prepared using low molecular weight components or a high wax content, application viscosity may suffer and there is a loss of adhesive properties, e.g., toughness, heat resistance and, often, specific adhesion to a substrate. While softer or more amorphous components may be added in order to improve adhesion, these components reduce the effective heat resistance.
There continues to be a need in the art for improved hot melt adhesives that can be applied at temperatures of from about 100xc2x0 C. to about 135xc2x0 C., have good bond strength with exceptional toughness, good heat resistance and acceptable application viscosity, even when prepared with low molecular weight components for application at a low temperature. The current invention fulfills this need.
The invention provides a hot melt adhesive comprising an ethylene vinyl acetate (EVA) polymer having a melt index (MI) of 550 grams/10 minutes or higher. Preferred embodiments will also comprise a paraffin wax, a rosin derived tackifier and, optionally, an aromatic hydrocarbon tackifier. The adhesive can be applied at a low temperature but still imparts heat stress normally associated with higher molecular weight polymers and has a longer pot life (less thermal degradation), exhibits less stringing, and is less damaging to heat sensitive substrates.
One aspect of the invention is directed to a hot melt adhesive comprising from about 20 to about 60 weight percent of at least one EVA polymer containing from about 22 to about 40% by weight of vinyl acetate and having a MI of 550 grams/10 minutes or higher, from about 20 to about 40%, more preferably from about 25 to about 35%, by weight of a paraffin wax having a melting point of from about 60 to about 68xc2x0 C. and an oil content of less than about 0.5%, more preferably from about 0 to about 0.2%, from about 25 to about 60 weight % of a rosin derived tackifier and from 0 to about 35 weight % of an aromatic tackifier.
In another aspect of the invention, the EVA polymer is replaced, either in whole or in part, by an ethylene 2-ethyl hexyl acrylate (EEHA) polymer. Preferred are EEHA polymers containing from about 33 to about 40% 2-ethyl hexyl acrylate.
The rosin derived tackifiers preferred for use will generally have average softening points ranging from about 85xc2x0 C. to about 130xc2x0 C., more typically from about 100xc2x0 C. to about 125xc2x0 C., will have a weight average molecular weight greater than about 1000, will have an acid number of less than about 20 and will have a viscosity at 125xc2x0 C. of greater than about 10,000 cp.
Particularly preferred compositions will comprise about 30% EVA polymer containing about 28% by weight of vinyl acetate and having a MI of about 800 grams/10 minutes, about 30% of a paraffin wax having a melting point of about 64xc2x0 C. and an oil content of less than about 0.5%, and about 40% of a rosin ester having a minimum softening point of about 100xc2x0 C.
The adhesives of the invention are typically formulated to have an application viscosity of between about 500 cp and 1750 cp at 120xc2x0 C. The resulting compositions produce good adhesion and impart heat resistance comparable to formulations utilizing low MI EVA polymers. The compositions impart heat resistance nearly (90%) or equivalent to the melting point of the wax.
Another aspect of the invention is directed to a method of sealing and/or making or forming a case, a carton, a tray, a box or a bag. These methods comprise using a hot melt adhesive comprising an adhesive polymer, a wax and a tackifier, more specifically an adhesive polymer comprising an EVA polymer having a MI of at least about 550 grams/10 minutes.
Still another aspect of the invention is directed to an article of manufacture comprising an adhesive polymer, wax and rosin tackifier. Preferred articles include cartons, cases, trays, bags and like packaging articles used for packaging products that are formed using a hot melt adhesive comprising an EVA polymer having a MI of 550 grams/10 minutes or more, a wax and a rosin tackifier. The packaging article may comprise cardboard or paperboard that has been adhered by such hot melt adhesives. In another embodiment, the adhesive is pre-applied to the article, e.g., carton, case, tray or bag during the manufacture thereof, and prior to packaging of a product. Other articles include but are not limited to books and nonwovens such as diapers and the like.
Yet another aspect of the invention is directed to packaged articles contained within a carton, case, tray or bag, such as a packaged food, wherein the carton, case, tray or bag comprises an adhesive comprising an EVA polymer having a MI of 550 grams/10 minutes or more, a wax and a rosin tackifier.
Another aspect of the invention is directed to a process for bonding a substrate to a similar or dissimilar substrate comprising applying to at least one substrate a molten hot melt adhesive composition and bonding said substrates together, said hot melt adhesive comprising an EVA polymer having a MI of 550 grams/10 minutes or more, a wax and a rosin tackifier.